The presence of large deposits of oil shale in the Rocky mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting and expanding a portion of the remaining formation to form a fragmented, stationary, permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of a combustion zone feed containing oxygen downwardly into the combustion zone to advance the combustion zone downwardly through the retort. The combustion zone feed can contain steam provided by a steam generator to improve efficiency of retorting. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying combustion zone feed downwardly into the combustion zone, the combustion zone is advanced downwardly through the retort.
The effluent gas from the combustion zone comprises combustion gas and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid hydrocarbon products and retorted oil shale containing residual solid carbonaceous material.
The liquid products and gaseous products are cooled by cooler oil shale fragments in the retort on the advancing side of the retorting zone. An off gas containing combustion gas generated in the combustion zone, product gas produced in the retorting zone, gas from carbonate decomposition, and any gaseous combustion zone feed that does not take part in the combustion process is withdrawn to the surface. Liquid hydrocarbon products, together with water produced in or added to the retort, are also withdrawn to the surface as a liquid product stream through an access tunnel, drift or shaft.
During the retorting process, oxygen in the combustion zone feed can pass into the retorting zone, resulting in oxidation of hydrocarbon products produced in the retorting zone. This has the undesirable result of reducing hydrocarbon yield.
Therefore, there is a need for a process for retorting oil shale which minimizes oxidation of hydrocarbon products produced during retorting.